1. Field of the Invention
The technical field of the present invention relates to a semiconductor device and a manufacturing method thereof. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
In recent years, metal oxides having semiconductor characteristics (hereinafter, referred to as oxide semiconductors) have attracted attention. Oxide semiconductors may be applied to transistors (see Patent Documents 1 and 2), for example.
There are many types of transistors. For example, transistors may be classified as a bottom gate-type structure and a top gate-type structure according to the positional relationship among a substrate, a gate, and a channel formation region. A transistor structure having a gate placed between a channel formation region and a substrate is called a bottom gate-type structure. A transistor structure having a channel formation region placed between a gate and a substrate is called a top gate-type structure.
In addition, transistors may be classified as a bottom contact type and a top contact type according to connection portions of a source and a drain with a semiconductor layer in which a channel is formed. A transistor with a structure where the connection portions of a source and a drain with a semiconductor layer in which a channel is formed is placed on a substrate side is called a bottom contact type. A transistor with a structure where the connection portions of a source and a drain with a semiconductor layer in which a channel is formed is placed on a side opposite to a substrate (that is, a counter substrate side in a liquid crystal display device) is called a top contact type.
Types of transistors can be classified as a BGBC (bottom gate bottom contact) structure, a BGTC (bottom gate top contact) structure, a TGTC (top gate top contact) structure, and a TGBC (top gate bottom contact) structure.